Scientists think that the finding might help solve the mystery of how species divide into distinct groups.

Above: The killer sperm in closeup

The study mated pairs of different species of worms (all in the Caenorhabditis genus) and observed what happened when the infected worms mated. What they saw was horrifying (if you're a worm) -- the sperm broke through the female's uterus and destroyed the eggs, and in some cases when further and caused fatal injuries to the worm itself.

"We found that the incidence of sterility depends strongly on which species pair is crossed,” co-author Asher Cutter to National Geographic.

The scientists aren't quite sure how or why the effect occurred. It is possible that compounds in the fluid relaxed the muscles that normally keep the sperm inside the female's reproductive organs. It is also possible that other mechanisms account for the 'killer sperm's' aggressive nature. The overall theory is that the female worms of a specific species are only tuned to deal with the aggressive sperm of that one species.

"Punishing cross-species mating by sterility or death would be a powerful evolutionary way of maintaining a species barrier," said Eric Haag of the University of Maryland.

The full study can be read here. Wired reports that a follow-up will look at whether any offspring that are produced by cross-species mating can breed with other species.